Learning outcomes

By the end of this lecture/handout, you should be able to:

1. Distinguish lithostratigraphic vs lithodemic units (and when to use each).
2. Explain why the Arabian Shield requires a tailored code for mapping and nomenclature.
3. Apply a decision workflow to classify units in arc terranes and deformed belts.
4. Recognize how terranes and shear/fault systems constrain unit names and correlations.
5. Use correct ranks, naming rules, and what-not-to-do pitfalls.

What is the ASLLC?

The Arabian Shield Lithostratigraphic and Lithodemic Code (ASLLC) is a set of rules and procedures for classifying, ranking, and naming geological units of the Precambrian Arabian Shield.

It provides: - standardized definitions of rock units, - clear hierarchical ranks (formation, lithodeme, suite, etc.), - consistent naming conventions, - a framework compatible with modern digital geological mapping.

Why was it established?

The ASLLC was developed to support the nationwide remapping of the Arabian Shield at 1:100,000 scale, because:

• Older stratigraphic schemes were largely informal and inconsistent
• Many units mixed different rock types, ages, and origins
• Classical stratigraphy is inadequate for arc-dominated, deformed Precambrian crust
• Intense deformation obscures original stratigraphic relationships
• Modern mapping requires database-ready, reproducible unit definitions

Why a stratigraphic code?

A stratigraphic code is a rulebook for:

• defining rock units,
• assigning rank and category,
• naming units consistently,
• ensuring reproducible mapping and communication.

In modern mapping programs, the code also ensures database interoperability: unit codes link maps to geochemistry, geochronology, structural data, and field observations.

Why ASLLC? The Arabian Shield mapping problem

The Arabian Shield is dominated by Neoproterozoic arc-related rocks formed during accretion and amalgamation.

Common challenges in the Shield:

• intimate relations between plutonic, subvolcanic, and volcanic facies,
• many lithologically similar arc packages in a condensed time window,
• limited crosscutting relations at map scale,
• localized but intense deformation (e.g., major shear zones),
• the need to remap consistently at 1:50,000 field scale, published at 1:100,000.

The big idea: two categories of units

ASLLC deals with formal subdivision of two categories of rock units:

1. Lithostratigraphic units (generally layered/stratiform)
2. Lithodemic units (generally non-layered/non-stratiform)

A major additional development is a structural scheme for highly deformed rocks, and a mixed scheme using “complex” when mapping components separately is impractical.

Decision question (the most important in ASLLC)

Which scheme do I use? Ask:

1. Are primary depositional/volcanic structures preserved?
2. Can you establish way-up / stratigraphic order?
3. Does the body generally obey Law of Superposition?
4. Is deformation pervasive enough to transpose original features?
5. Can components be mapped separately at the working scale?

Outcome:

• Yes → Lithostratigraphic
• No → Lithodemic
• Mixed & unmappable → Complex
• Transposed by high strain → Structural units (structural lithodemes + zones/systems)

Lithostratigraphy: what it is

Lithostratigraphy classifies stratiform volcanic and sedimentary rocks based on:

• lithological character,
• relative stratigraphic position.

Typical properties:

• layered/tabular,
• primary structures preserved,
• superposition generally applies.

Low-grade metamorphic equivalents can still be lithostratigraphic if primary structures and way-up criteria are preserved.

Lithostratigraphic hierarchy (ranks)

From highest to lowest:

• Supergroup
• Group
• Subgroup
• Formation (fundamental unit)
• Member
• Bed (sedimentary) / Flow (volcanic)

Formation is the key mapping unit: identifiable, and mappable.

Formation: definition & practical mapping rules

A formation is a body of rock defined by lithology and stratigraphic position that is:

• mappable at the Earth’s surface, or
• traceable in the subsurface.

Practical points:

• Thickness is not the criterion, but mappability is.
• Very thin but important units can be shown as a line, but excessive proliferation is discouraged.
• Interbedded volcanic + sedimentary rocks may be in one formation if they form a coherent accumulated package.

Arabian Shield context: “supracrustal rafts” and correlation

In arc terranes, supracrustal sequences commonly occur as isolated rafts intruded by related plutons.

ASLLC guidance:

• Avoid proliferation of names: correlate rafts into a single formation where defensible.
• Correlation evidence:
  • superposition is obeyed,
  • significant similar lithic components,
  • similar stratigraphic order.

Lithodemics: what it is

Lithodemics classifies rock bodies that are:

• intrusive,
• highly metamorphosed/metasomatized,
• pervasively deformed,
• or otherwise non-stratiform.

Key contrast:

• Lithodemic units generally do not obey superposition.
• They are defined by observable lithic features and mapping practicality.

Lithodemic hierarchy (ranks)

From highest to lowest:

• Supersuite
• Suite
• Subsuite
• Lithodeme (fundamental unit)

Notes:

• Units below lithodeme rank are informal.
• Suites express natural relationships among lithodemes (broadly cogenetic).

Lithodeme: definition and naming

A lithodeme is the fundamental lithodemic unit:

• intrusive / high-grade metamorphic / pervasively deformed rock body,
• typically non-tabular,
• primary depositional structures absent,
• mappable at the surface.

Naming pattern: Geographic term + lithic term
Examples: Malik Granite, Abt Schist, Dumaym Diorite.

Do not add “Lithodeme” to the name (e.g., not “Abt Lithodeme”).

Suite and Supersuite: what they mean

A Suite groups ≥2 associated lithodemes of the same class (intrusive or metamorphic).

• Comparable (cartographically) to a lithostratigraphic Group.
• A Subsuite may group a subset of lithodemes in a suite if useful.

A Supersuite groups ≥2 suites/complexes with broader natural relationship.

Important modernization:

• “Igneous complex” (old usage) is obsolete for grouping associated intrusives.
• Use Intrusive Suite / Metamorphic Suite.

Complex: when you are allowed to use it

A Complex is used only when mapping each component separately is impractical at the working scale.

Three accepted complex types:

1. Complex (generic): mixed classes and/or mixed ages where components cannot be separated.
2. Volcanic Complex: diverse, cogenetic extrusive volcanic + volcano-sedimentary assemblage (often rank-like a subgroup).
3. Intrusive Complex: variable, coeval intrusive assemblage that cannot be separated (often rank-like a subsuite).

Key caution: - Do not use “complex” simply because rocks look complicated. - It is a cartographic and subdivision tool, not a shortcut.

Highly deformed rocks: the structural scheme (major ASLLC innovation)

When deformation transposes original fabrics and primary features such that:

• the protolith cannot reliably be classified as formation/lithodeme,
• or units are tectonically mixed (mélange),

ASLLC uses a structural hierarchy:

• Structural lithodeme
• Shear Zone or Fault / Fault Zone
• Shear Zone System or Fault System

These replace older vague “structural complex” concepts.

Definitions: shear zone vs fault vs fault zone

• Shear Zone: planar zone of localized strong deformation; includes ductile zones and brittle faults.
• Fault: discrete brittle discontinuity with displacement, mapped as linear feature.
• Fault Zone: polygonal zone of closely spaced faults/dominantly brittle deformation.

Systems (rank above): - Shear Zone System / Fault System must include ≥2 named zones/faults and reflect a broadly coeval tectonic regime.

Transitional units in deformation zones (naming rule)

It is common for a unit to grade from undeformed → deformed → fully transposed.

Rule of thumb:

• If protolith is recognizable → keep geographic name, add descriptor (e.g., sheared/foliated), assign a distinct code/polygon.
• If fully transposed/mixed and protolith not identifiable at mapping scale → define a new structural lithodeme with new geographic name.

Boundaries are often somewhat arbitrary and decided by the mapping team.

Terranes and tectonostratigraphic position: why unit names may be terrane-limited

The Arabian Shield is a mosaic of originally allochthonous terranes.

Implication for naming/correlation:

• Units formed before terrane amalgamation are typically restricted to their terrane/boundary zones.
• Units formed after amalgamation that span multiple terranes should share stratigraphic designations across those terranes.

This protects against false correlation of unrelated arc packages.

Formal unit requirements (what a proper unit description must include)

To establish or modify a formal unit, documentation should include:

• formal name, category, rank, geographic name basis,
• stratotype/type locality (GPS + access),
• tectonostratigraphic position and age constraints,
• lithologic description (field + petrography; volcanic geochem where needed),
• boundaries (sharp/gradational; criteria),
• thickness and dimensions,
• correlation rationale (data vs inference),
• history of naming/previous usage,
• representative photos and (for stratiform units) measured sections.

Naming rules you must follow

Formal names are compound.

Lithostratigraphic units

• Geographic + rank: Qarfa Formation
• Geographic + lithic + rank (optional): Qarfa Rhyolite Formation
• Groups do not include lithic term: Bayda Group

Lithodemic units

• Lithodeme: Malik Granite (no “formation”, no “lithodeme” word)
• Suite: Abanat Intrusive Suite / Someplace Metamorphic Suite
• Complex: Someplace Complex, Someplace Intrusive Complex, Someplace Volcanic Complex

Capitalization: - First letter of each word in formal unit names is capitalized.

What ASLLC explicitly does NOT do (avoid these errors)

ASLLC is not:

• chronostratigraphy (time units are not the basis of definition),
• magnetostratigraphy / biostratigraphy / allostratigraphy, etc.,
• a tectonic classification system.

Terms such as arc, ophiolite, belt, pluton, ring complex are generally informal descriptors. They may appear in legends or free-text notes but are not formal database unit terms.

Why this matters for digital mapping and exploration

ASLLC is designed for a modern digital mapping workflow:

• unique unit codes enable relational linking between:
  • geology polygons,
  • geochemistry,
  • geochronology,
  • structures,
  • field stations and samples.

Practical value: - clearer regional synthesis, - improved reproducibility, - stronger framework for mineral and energy exploration targeting.

Worked decision example (conceptual)

Scenario: An arc volcanic sequence occurs as rafts, intruded by tonalite–granodiorite, cut by a major shear zone.

Classification:

1. Stratiform volcanics + sediments with preserved bedding → formations (lithostratigraphy).
2. Tonalite–granodiorite pluton with gradational internal contacts → lithodeme with compound lithic term if needed.
3. A mixed contact zone that cannot be separated at mapping scale:
   • if multiple coeval intrusions → intrusive complex
   • if mixed classes (intrusive + supracrustal) → complex
4. Where transposed in the shear zone:
   • identifiable protolith → same geographic name + structural descriptor + new code
   • unidentifiable protolith → structural lithodeme, within a shear zone (and potentially a shear zone system)

Arabian Shield Lithostratigraphic and Lithodemic Code (ASLLC)

Image generated by NotebookLM
Saudi Geological Survey Special Publication Report SGS-SP-2024-2 (ASLLC, 2024).